Within recent years flexographic inks and plates have been developed to produce very accurate pictorial reproduction of a great variety of subject matter. For example, photographs depicting pastoral scenes and still lifes. In fact, process flexo printing compares favorably with the results achieved by rotogravure processes. Development of flexoprinting to its present state has been the result of a variety of evolutionary factors, not the least of which are the high degree of plate quality and flexographic process inks. While the inks must have physical properties to fulfill a variety of operating conditions, one of the most important properties is that of resoluability. Failing to meet this criteria causes build-up of inks in the cells of the anilox roll resulting in poor definition of the dot structures and, of course, the quality of the final product. Resoluability is also a significant factor in determining printing fidelity and speed.
Two approaches may be used in metering the ink on the anilox roll. One involves use of a doctor blade and the other involves a two-roll system which may consist of a fine line anilox roll and a hard rubber roll having a durometer on the shore A scale of 90. It is usual practice, preferred by many converters, to use a doctor blade oriented at a reverse angle since it produces better definition and constant density of color.
One of the problems arising with the use of processed inks is that of maintaining a pool of ink for distribution to the surface of the anilox roll regardless of speed and the physical properties of the ink. At certain web speed the gap between the anilox roll and the fountain roll did not maintain a substantially uniform pool of ink which resulted in storing certain cells of the anilox roll. By adjusting the spacing between the anilox roll and the fountain roll, the condition was improved but such an approach does not provide a viable operating environment solution. For example, and as will be explained in greater detail hereinafter, under certain operating conditions it has been found that a more narrow gap between the fountain roll and the anilox roll will produce good printing results at higher web speed.